1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming apparatus and a control method for same, and more particularly, to an image forming apparatus such as a laser beam printer, a LED printer, a facsimile machine, and the like, and a control method for the above-described image forming apparatus.
2. Discussion of the Related Art
Related-art image forming apparatuses include multiple image forming units to form different single color images that correspond to respective colors including black.
In particular, one related-art image forming apparatus includes a direct image transfer position and a secondary image transfer position.
A black image formed by a black image forming unit is transferred at the direct image transfer position directly onto a recording medium.
By contrast, single color images other than black, which are formed at the corresponding image forming units of different colors (typically yellow, cyan, and magenta) and transferred as a composite color image onto an intermediate transfer belt in a primary image transfer operation, are transferred again at the secondary image transfer position to transfer the composite color image onto a recording medium in a secondary image transfer operation.
The secondary image transfer position is located upstream from the direct image transfer position in a direction of conveyance of recording medium. The intermediate transfer belt is extended around multiple rotatable rollers and is rotated by a drive roller that is one of the multiple rollers.
Further, such related-art image forming apparatus includes a sheet conveyance belt. Similarly to the intermediate transfer belt, the sheet conveyance belt is extended around multiple rotatable rollers to carry and convey the recording medium to the direct image transfer position and then on to the secondary image transfer position.
In this related-art image forming apparatus, the sheet conveyance belt conveys the recording medium through the direct image transfer position and the secondary image transfer position to overlay first the color images transferred from the secondary image transfer position onto the recording medium and then the black image transferred from the direct image transfer position onto the recording medium to form a full-color image on the recording medium.
At present, image forming apparatuses currently on the market that are capable of producing color images are used at a rate of 70% to 80% to produce monochrome (black-and-white) images. Since black toner is consumed when producing full-color images as well as monochrome images, in the interest of saving resources and reducing costs it is desirable that an amount of black toner consumed when producing full-color images be reduced.
Similar to the above-described related-art image forming apparatus, toner image transfer efficiency increases more by transferring a black image formed on an image carrier of a black image forming unit onto the recording medium directly than by transferring the black image from the image carrier onto the recording medium indirectly, that is, via the intermediate transfer belt. Therefore, a smaller amount of black toner can be consumed in forming a black image on an image carrier incorporated in the black image forming unit when transferring the black image from the image carrier onto the recording medium directly than when transferring the black image from the image carrier onto the recording medium indirectly via the intermediate transfer belt.
The color images produced in the above-described related-art image forming apparatus are transferred onto the recording medium conveyed by the sheet conveyance belt at the direct image transfer position and the secondary image transfer position. The different transfer positions of the color images transferred onto the recording medium can easily cause positional deviation between the color images on the recording medium.
Moreover, the relative positions of the transfer positions in the direction of conveyance of the recording medium does not affect this susceptibility to positional deviation between the color images on the recording medium. Thus, in the above-described related-art image forming apparatus, the secondary image transfer position is located upstream from the direct image transfer position in the direction of conveyance of the recording medium. However, even when the secondary image transfer position is located downstream from the direct image transfer position in the direction of conveyance of the recording medium, a similar problem to the above-described problem of positional deviation between the color images on the recording medium may still occur.